This invention relates to the field of acoustic signal processing and more specifically to acoustic signal filtering. It is particularly useful in hearing protection applications, for instance a noise protection ear plug, where a tradeoff exists between the level of hearing protection and the ease of communication.
Every day, people in all types of different situations are exposed to sound. This sound may range in quality, from quiet and soothing to loud and disturbing. Sound is caused by vibrations. When a person speaks, air is forced across the vocal cords causing them to vibrate. Similarly, the speakers in a stereo system vibrate to produce sound.
The xe2x80x9csoundxe2x80x9d can be defined as alterations in air pressure that are perceived by hearing organs with respect to time. These alterations consist in changes in the air pressure from a low pressure zone, low density of air molecules, to a high pressure zone, high density of air molecules, and vice versa. As an example, in a quiet room with no perceivable sound there exists a steady-state of air pressure as registered by the hearing organ. When a sound is generated, alterations in the steady-state air pressure are created. If the sound originates from a loudspeaker, the diaphragm of the loudspeaker moves outward compressing the molecules of air, creating a high pressure zone. Immediately following this, the speaker diaphragm is moved inward resulting in a rarefaction of the air molecules. This action creates the low pressure zone that follows the high pressure zone.
Sound travels via the process of molecules of air interacting with each other, thus transferring their energy from one molecule to the next. The radiation of sound implies an energy transfer through the molecules of air, resulting in a moving high pressure (compression) zone traveling at 1130 feet/second.
Two important points should be noted, relevant to the field of acoustics, which arise in a variety of different situations particularly those involving speech:
the need to extract, and possibly amplify, one particular component of a sound, for instance a voice signal superimposed with noise;
the need to attenuate a particular sound, for purposes of hearing protection, environment control and others, in situations where the characteristics of pitch, intensity and timbre of the sound are uncontrollable and unfavorable.
An existing solution is the acoustic filter. An acoustic filter is a device usually employed to reject or attenuate sound in a particular range of frequencies, while passing sound in another range of frequencies. One example of the use of such a device is within noise protection ear plugs, used to attenuate surrounding noise, whereby the acoustic filter attenuates a desired portion of the sound spectrum in order to facilitate speech reception.
Unfortunately, existing acoustic filter applications, specifically those directed to hearing protection, do not address the tradeoff between fidelity of communication and the level of hearing protection. This constitutes a drawback that the present invention aims to alleviate.
An object of this invention is to provide an improved acoustic signal filter.
Another object of this invention is to provide a novel method for performing selective acoustic signal filtering.
As embodied and broadly described herein, the invention provides an acoustic signal shaping device, comprising:
an input port for receiving an input acoustic signal to be processed;
an acoustic signal processing device for producing an output acoustic signal in response to the input acoustic signal, said acoustic signal processing device including:
1) a diaphragm within a path of propagation of the input acoustic signal, said diaphragm manifesting vibration in response to interaction with the input acoustic signal to generate the output acoustic signal;
2) said acoustic signal processing device characterized by a frequency response such that the output acoustic signal spectrum level manifests predetermined differences with respect to the input acoustic signal spectrum level;
an output port for releasing the output acoustic signal produced by said acoustic signal processing device.
In a specific example, the acoustic signal shaping device forms an acoustic filter used in a noise protection ear plug. The filter is able to provide an acoustic shunt to reduce the ear protector""s attenuation in a desired portion of the sound spectrum. This is to facilitate speech reception while maintaining the desired noise protection characteristic of the ear plug.
The acoustic filter comprises two acoustic ports, and an acoustic signal processing device. The acoustic input port receives the input acoustic signal in the form of sound waves, while the acoustic output port releases the output acoustic signal from the filter. The acoustic signal processing device itself comprises an acoustic diaphragm that vibrates when exposed to the input acoustic signal. In addition, the acoustic signal processing device includes an energy dissipation component that is magnetically, coupled to the diaphragm. More specifically, the diaphragm is structurally coupled to a permanent magnet and causes this magnet to oscillate when the diaphragm vibrates. In turn, this creates an oscillating magnetic field. In a specific example, the energy dissipation component is an electrical circuit behaving as a passive filter. Such passive filter could be a band stop filter. The electrical circuit includes an inductor located in proximity to the permanent magnet to capture the oscillating magnetic field. This oscillating magnetic field induces a an oscillating electrical current in the resonant circuit. When the frequency at which the diaphragm vibrates is close or is the same as the resonant frequency of the resonant circuit, an energy transfer occurs between the diaphragm and the resonant circuit. As a result, the diaphragm will pass the energy contained in the input acoustic signal in this frequency range to the resonant circuit, rather than in the output acoustic signal. This mechanism provides the filtering or sound conditioning behaviour of the acoustic filter.
The signal conditioning properties of the acoustic filter can be varied by altering the response characteristics of the energy dissipation component. This provides the acoustic filter with adjustability.
As embodied and broadly described herein, the invention provides a method for performing selective acoustic signal filtering, said method comprising the steps of:
receiving an input acoustic signal;
exposing a diaphragm to the input acoustic signal to induce vibrations in said diaphragm, the vibrations causing generation of an output acoustic signal;
controlling a vibration characteristics of said diaphragm such that the output acoustic signal spectrum level manifests predetermined differences with the input acoustic signal spectrum level;
releasing the output acoustic signal.
As embodied and broadly described herein, the invention further provides an acoustic filter, comprising
a diaphragm having a first face and a second face, said first face being exposed to an input acoustic signal to be processed, said diaphragm manifesting vibration in response to interaction with the input acoustic signal to generate the output acoustic signal at said second face;
a magnet coupled to said diaphragm for moving with said diaphragm when said diaphragm manifests vibration, thereby causing generation of a fluctuating magnetic field;
an energy dissipation device including an electrical coil located in the fluctuating magnetic field to generate in response to the fluctuating magnetic field a variable electric signal, said energy dissipation device being capable of attenuating the electric signal at a level depending upon a frequency of said electric signal.